1. Field of the Invention
The present invention provides an irreversible device for preventing access to an integrated circuit, which can be used in every field where it is necessary, to irreversibly prohibit access to certain zones or functions of the integrated circuit.
This device is particularly important for restricting the internal accessibility of an integrated circuit after tests, when it leaves the production line. Indeed, after manufacture, an integrated circuit has to be tested comprehensively to an extent which it is constantly improving. In particular, the internal accessibility of the integrated circuit must be the maximum during the test. However, this accessibility must thereafter be restricted, for a user of the circuit should have access, after these tests, only to a so-called applicative layer. It is therefore necessary to prohibit certain paths of access.
Also, a user of a programmable integrated circuit should be able to adapt this integrated circuit to a particular purpose with the programming of an application. It may then be particularly important to prohibit modifications, or even the external reading, of the pieces of information contained in certain memory zones. It may also be important to take precautions against external disturbances, if any, that might affect these memory zones. Locking devices enable such protection. These devices are, for example, required in integrated circuits provided with security systems, such as the circuits of computerized money products.
2. Description of the Prior Art
In the prior art, when these locking devices leave the production line, they are in an unlocked state and are equivalent to closed switches. These devices are programmable only once, irreversibly. Once programmed, after the tests, they are equivalent to open switches. These devices are even integrated into the circuit, so that there is no possibility of short-circuiting them again. Their states are taken into account in the logic equations of access to the memory zones or the functions that they control.
There are known programmable devices with programmable fuses. Each fuse, made for example on a very thin layer of polycrystalline silicon, is supplied with current by a transistor. When a current flows for a certain time, local heating occurs, prompted by the very thin section of the fuse. This heating is enough to make the metal vaporize and the fuse is said to be burnt out. In fact, sometimes the fuse does not get burnt out. In practice, a device with fuses is not very reliable.